The local excess air factor and the average excess air factor are further explained with reference to FIGS. 12A and 12B.
When air in a combustion chamber (a total air in a cylinder) is defined as “A”, air forming an air-fuel mixture is defined as “B”, and fuel forming the air-fuel mixture is defined as “C”, then the local excess air factor is represented by “(B/C)/λ”, whereas the average excess air factor is represented by “(A/C)/λ”, wherein A includes B.
It is important for the gas fuel engine operating with hydrogen to reduce production of NOx.
Prior arts for reducing NOx are disclosed, for example, in Japanese Unexamined Patent Publications No. H5-118253, No. H6-241077, and No. 2004-76686, as well as Japanese Examined Publication No. H4-37264B.
Japanese Unexamined Patent Publication No. H5-118253 discloses a technology that an operating area of an engine is restricted to such an area, in which the local excess air factor λ is not less than 2, since this prior art makes use of a fact that almost no NOx is produced in the area of the local excess air factor λ not less than 2.
However, if the operating area of the engine is restricted to the above area, an output power from the engine would be insufficient. Therefore, when the engine of this kind is used for a hybrid car, an electric motor is used for supplement a necessary driving power which would not be obtained by the output of the engine alone. And the engine of this kind may not be applied for a motor vehicle, by itself, due to lack of the output power.
Japanese Unexamined Patent Publication No. H6-241077 discloses a technology that a mixture of fuel and air is formed by a pre-mixture forming or a direct fuel injection, wherein the pre-mixture forming and the direct fuel injection is switched from one to the other depending on the average excess air factor λ, namely depending on whether the average excess air factor λ is higher or lower than a predetermined local excess air factor λ0 (1<λ0<2, see FIG. 2), so that production of NOx is reduced.
However, the operating area at or around the excess air factor λ0 is the operating area, in which a larger amount of NOx is produced. Accordingly, the production of NOx can not be sufficiently reduced either by the pre-mixture forming or the direct fuel injection.
Japanese Unexamined Patent Publication No. 2004-76686 discloses a technology that the average excess air factor λ is temporally controlled to be lower than 1 (λ<1) when an accumulated amount of NOx in a NOx catalyst exceeds a predetermined value, so that the accumulated NOx is deoxidized and exhausted from the engine.
However, in the case that NOx is accumulated quickly in the catalyst and the control of the excess air factor λ below 1 shall be often carried out (at a shorter interval), a fuel consumption ratio is decreased, because additional fuel which would not contribute in generating engine output is used.
Japanese Examined Patent Publication No. H4-37264 discloses a technology that a combustible mixture (the mixture is surely ignited) is formed at or around a spark discharge portion (a spark gap for spark discharge) by dividing the fuel injection into multiple number of injections.
Even when the mixture can be surely ignited, the production of NOx can not be always reduced, because a rich air-fuel mixture (the local excess air factor λ is smaller than 2) may locally but widely exist in the entire space of the combustion chamber.